Xenotransplants are typically rejected within minutes to hours in untreated recipients. This early rejection is called hyperacute rejection and results from recipients having preexisting xenoreactive natural antibodies which bind to antigens of the endothelial cells of the donor organ, thereby activating the complement cascade. This ultimately results in the formation of a membrane attack complex which activates the endothelial cells or penetrates the cell membrane, resulting in cell death and necrosis of the xenograft. The activation of endothelial cells is devastating, resulting in thrombosis, inflammation and rejection of the transplanted tissue.
Thus, complement activation plays a major role in the pathogenesis of hyperacute rejection of xenografts, and several strategies for combating hyperacute rejection have been developed which are aimed at preventing activation of the complement cascade, including the production of transgenic pigs capable of expressing genes for human complement regulatory proteins on organs intended for transplantation into humans (Nature Medicine, 1, 869-873, 1995). These strategies, the role of complement, and hyperacute rejection have recently been reviewed (Cozzi and White, Nature Medicine, 1, 964-6, 1995; Ryan, Nature Medicine, 1, 967-8, 1995; Parker, et al., Immunology Today, 17, 373-378, 1996; White, Xeno, 4, 46-49, 1996).
These strategies can prevent immediate graft loss associated with hyperacute rejection, but a delayed xenograft rejection (or acute vascular rejection) still occurs a few days later (Hancock and Bach, Xeno, 2, 68-72, 1994) in which, the evidence indicates, antibody-dependent cell-mediated cytotoxicity plays a prominent role (Baldwin, et al., Transplantation, 59, 797-808, 1995; Schaapherder, et al., Transplantation, 57, 1376-82, 1994). In many of the strategies designed to block hyperacute rejection, the complement cascade is regulated at the level of C3 or at more distal points, so that deposition of the early binding components (C1q, C1r, C1s, C2, C4) still proceeds. Also, with any of the strategies, C1q will still bind to certain immunoglobulins (Igs), especially IgM and the IgG1 and IgG3 subclasses in humans. Thus, C1q (C1) binds with activation of C1, C2 and C4, and these early complement components are capable of producing changes characteristic of delayed xenograft rejection. Furthermore, many immunocompetent cells which play a role in rejections, such as monocytes, macrophages, platelets, neutrophils and B lymphocytes, bind to C1q bound to the graft through C1q receptors expressed on their surfaces (Baldwin et al., Transplantation, 59, 797-808, 1995). Many of the cellular events triggered by these interactions can be harmful to the graft (Fryer, et al., Surgical Forum, 46, 396-7, 1995; Ghebrehiwet, B. et al., Behring Inst. Mitt., 93, 236-240 ,1993).
Hyperacute rejection and acute rejection of allografts occur in recipients having preexisting antibodies which bind to antigens of the graft, particularly to histocompatibility antigens. Hyperacute and acute rejection occur by mechanisms analogous to those seen in xenograft rejection discussed above, including C1q binding to antibodies deposited on the graft resulting in complement activation and the binding of immunocompetent cells to C1q. See Baldwin et al., Transplantation, 59, 797-808 (1995). C1q-mediated processes are possibly involved in chronic rejection of allografts, as well. See id.
Inflammation in some diseases, such as rheumatoid arthritis, has been associated with the deposition of immune complexes in tissues and the activation of the complement cascade. The binding of C1q to immune complexes deposited in the tissues initiates the complement cascade, and the action of the complement components, alone or concurrently with other biologic molecules, ultimately leads to tissue damage. The binding of immunocompetent cells to C1q at sites of inflammation may also be involved in inflammatory processes.
It has previously been shown that a peptide derived from C1q could inhibit C1q binding to immunoglobulin (Baumann, Mass. and Anderson, B E, J. Biol. Chem., 265, 18414-22, 1990; U.S. Pat. No. 5,364,930). It was also found that this peptide could inhibit complement activation and prolong graft survival in an animal model (guinea pig heart to C6 deficient PVG rat model) (Baumann, Mass. Anderson, B E and Fryer, J P, "A Synthetic Peptide and its Uses", U.S. patent application Ser. No. 08/628,383, filed, Apr. 5, 1996), now U.S. Pat. No. 5,923,483.
C1q binds to the CH2 domain of IgG, and several sequences of the CH2 domain are potentially involved (Duncan, Ariz. Winter, G, Nature, 332, 738-9, 1988; Lubias, T J, Munoz, H, Erickson, B W, J. Immunol., 127, 2555-60, 1981; Boackle, R J Johnson, B J, Caughman, G B, Nature, 282, 742-3, 1979). Takada et al. (Takada, A, Shirahama, S, Takada, Y. Immunopharmacology, 9, 87-95, 1985) showed that peptides containing the sequence tryptophan-tyrosine (Trp-Tyr) and Trp-Tyr itself could inhibit complement activation. The Trp-Tyr sequence corresponds to residues 277-278 of the IgG CH2 domain.